Without appropriate physical activity, mobility and fitness are diminished in persons with incomplete spinal cord injury (iSCI), leading to degenerative physiological changes that negatively affect health and physical function. Conventional rehabilitation programs are designed to facilitate independence in performing functional tasks such as walking, but loss of muscle strength, endurance, and motor coordination limit the capacity of individuals with SCI to participate in traditional overground ambulation training regimens. While the use of land-based, body weight-supported treadmill training and computer-driven gait orthoses has led to gains in walking ability following iSCI, notable limitations associated with both gait retraining techniques have provided a strong impetus to explore new, activity- based therapies to improve walking performance in this clinical population. Against this backdrop, our primary research goal is to quantify the functional benefits of underwater treadmill training in adults with iSCI. Specifically, we hypothesize that underwater treadmill training (UTT) will improve daily ambulatory activity and reduce sedentary time, increase participation in life activities, and improve endurance walking performance in adults with iSCI. To test these hypotheses, 30 men and women with iSCI will be assigned to either an UTT group (n = 15; Group 1) or a control group (n = 15; Group 2) using a randomized crossover design. Participants in Group 1 will complete 16 weeks of underwater treadmill walking (3x/week) featuring systematic and gradual increases in exercise intensity and duration followed by 16 weeks during which retention of gains made during UTT will be evaluated. Conversely, participants in Group 2 will initially maintain a normal living routine for 16 weeks and then engage in UTT for 16 weeks. Results from the proposed study will allow us to establish the rehabilitative potential of underwater treadmill training in adults with incomplete spinal cord injury and better understand the synergistic effects of conditioning adaptations on physical activity, social and interpersonal aspects of disability, and aerobic health in this clinial population. Our ultimate objective in this work is to extend the access of underwater treadmill training, a technologically- advanced therapeutic option, beyond the research laboratory and into a variety of clinical and public settings, such as hospitals, trauma rehabilitation facilities recreational fitness centers, and retirement communities, where small, portable water-based treadmills can be easily transferred into pools.